Materials Performance

AUG 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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17 MATERIALS PERFORMANCE: VOL. 57, NO. 8 AUGUST 2018
Information on corrosion control and prevention
Study Explores Water Vapor Corrosion of Metals
at Atomic Level
S
cientists with several U.S. and Chinese
government agencies recently experi-
mented with oxide grow th at the atomic
level on a Ni-Cr alloy.
1
They say this
allowed them to model the process
through computer simulations to provide
insights into how water vapor could
change other materials, particularly at
elevated temperatures, and the pathways
to corrosion.
According to the researchers, while
engineers have long known water vapor
can accelerate the corrosion of metals
and alloys, the exact mechanisms behind
it have not been clear. In turn, this makes
the phenomenon diff icult to prevent.
However, by probing the atomic-level
reactions, researchers say they found that
the involvement of protons speeds up the
corrosion process.
"Understanding how water vapor such
as mist or steam corrodes metals and
alloys can help engineers keep industrial
systems working at peak performance
longer," says Chongmin Wang, a senior
research specialist at U.S. Department of
Energ y's Environmental Molecular Sci-
ences Laboratory (EMSL) (Richland,
Washington, USA) who helped lead the
study. "Armed with that knowledge, engi-
neers can also improve cataly tic conver-
sion processes and enhance ionic conduc-
tion in materials."
The researchers identif y steam gener-
ators, turbine engines, fuel cells, and cat-
alysts as examples of material applica-
tions where water vapor is present, be it
intentional or unavoidable.
In their study, the researchers used in
situ environmental transmission electron
microscopy (TEM) to examine a single
crystalline Ni-Cr alloy f ilm exposed to
both pure oxygen (O
2
) and water vapor
(H
2
O) at 350 °C. By contrasting the results,
they determined unique features from
H
2
O exposure.
For both environments, cuboid
nickel(II) oxide (NiO) crystals formed on
the alloy during oxidation, in which the
Ni and O atoms diffused to form an NiO
lattice, layer by layer. Compared with O
2
,
a unique feature of H
2
O oxidation was the
formation of vacancy clusters. These clus-
ters of vacancies, which originate when
an atom is missing from a lattice site, are
described as sub-nanometer cavities
formed by incorporating both Ni and O
vacancies. These cavities can merge with
other vacancy clusters and eventually
migrate to the surface.
In their study, with continued oxida-
tion in H
2
O, the vacancy clusters ty pically
migrated to the surface and created a
surface pit after ~174 s. The surface pit
then subsequently f illed up via the diffu-
sion and grow th of atoms and molecules
on the surface, leading to a f lat surface
after ~301 s. The process, which is not
observed during the grow th of NiO in
pure O
2
, then repeats as oxidation pro-
gresses. Hence, the vacancy formation
and migration in growing NiO in H
2
O
indicates a modif ied oxidation mecha-
nism, according to the researchers.
During the modif ied ox idation pro-
cess, H
2
O molecules were adsorbed and
chemically dissociated into negatively
charged hydrox ide (OH
−
) ions (anions)
and positively charged hydron (H
+
) ions
(cations) on the NiO surface. From there,
the O-H bonds were f urther broken to
form free ox ygen ions that ser ved as the
ox idizing species. According to the
researchers, H
+
could penetrate the NiO
lattice by overcoming a small diff usion
barrier. This led to the formation of
interstitial protons, H
i
, w ithin the NiO
lattice.
According to the study and subse-
quent modeling, the presence of H
i
enhanced vacancy generation, further
lowered the diffusion barrier, and thus
promoted the clustering of those vacan-
cies—which could lead to widespread
surface pitting.
Scanning TEM analysis also revealed
the morpholog y of the oxidizing Ni-Cr
surface in H
2
O and O
2
, respectively. A
thick ness contrast showed the oxide
layer formed in H
2
O was highly porous,
w ith an average pore size of ~5 nm after
30 min of oxidation. By comparison, oxi-
dation in pure O
2
did not lead to the for-
mation of pores.
"This indicates vacancy formation
and condensation are both enhanced in
The researchers identify turbine engines as one example of a material application where water
vapor is present and can possibly lead to corrosion.
Continued on page 18